A need exists to join thermoplastic components, such as those formed from glass fiber reinforced polypropylene and the like, to other components formed of the same or similar thermoplastic materials. In general, such joining is only achieved via adhesives, mechanical fasteners, laser welding, sonic welding and/or vibration welding, but each of these methods suffers from disadvantages.
One common problem with known methods to join plastic components is that low energy surface adhesives (LESA's) must be employed to adhesively join glass fiber reinforced polypropylene components. LESA's are expensive, typically require long cure times, and can produce undesired fumes while they cure. These factors contribute to high manufacturing costs for assemblies joined by LESA's.
Another common problem with known methods to join plastic components is that the joining of plastic components with mechanical fasteners can result in low strength joints, the mechanical fasteners only connect the components at specific locations, and have high installation labor costs. In addition, such known mechanical fasteners are subject to mechanical failure and can result in failure of the assembly of components.
An additional common problem with known methods to join plastic components is that the laser welding used to join plastic components requires that at least one of the components be at least partially transparent to the laser energy for the process to work. This limits the type of material, finishes, and colors of the components to be joined. In addition, laser welding is a linear process and thus long joints can take significant time to form as the laser must traverse the entire joint length. Additionally, laser welding techniques limit the amount of glass fiber which can be added to the thermoplastic material due to the scattering of the laser energy by the glass fibers. Thus, the mechanical properties of plastic components joined by laser welding can be unduly limited.
Vibration welding can also be employed to join plastic components. A common problem with known vibration welding is that it is limited to making planar joints which do not change profile. Thus, the use of vibration welding is often too limited for many desired assemblies. Sonic welding suffers from similar problems and is typically limited to making relatively short joints and has high associated equipment costs.
Recent interest has developed in joining plastic components, such as glass fiber reinforced polypropylene components and the like, by known resistive implant welding. In general, an electrically conductive implant is positioned between the two components to be joined and pressure is applied to the area of the components contacting the implant. An electrical current is then passed through the implant causing it to heat and melt the material of the components adjacent the implant. The melted portions of the components intermingle under the applied pressure. When the current is removed and the implant and components are cooled, a weld is formed between the components. Cycle times for the welding process of less than one minute can be achieved. While resistive implant welding has offered many advantages over other methods of joining plastic components, it has also suffered from disadvantages in that the placement and retention of the resistive implant between the components to be joined is difficult to achieve and/or labor intensive and has an undesirably long cycle time.
In general, a common method of resistive implant welding is described in, “Resistive Implant Welding of Glass Fiber Reinforced Polypropylene Compounds”, by Bates, Tan, Zak and Mah, published by the Society of Automotive Engineers, SAE Technical Papers, document number 2006-01-0332 and the contents of this paper are included herein, in its entirety, by reference. One presently known resistive implant is formed of a stainless steel mesh with wires of 0.009 inches in diameter woven in a plain weave of sixteen wires to an inch. This implant is positioned between the two surfaces of the components to be joined and pressure is applied to the components adjacent the implant. An electrical current is applied to the implant and the current passing through the stainless mesh generates heat which, in combination with the applied pressure, forms a weld between the two surfaces once the current is removed and the weld cools. While this known method can result in good welds between the components, it has proven to be difficult and labor intensive to position the implant in a desired position and to maintain it there during the heating and joining process. This is especially true if the weld is to be formed along join lines with complex geometries that can include curves, profile changes, etc.
Accordingly, there exists a need for an improved implant applicator and method for positioning and tacking a resistive implant for welding assemblies of plastic components which substantially obviates or mitigates these disadvantages and to help render resistive implant welding conducive to mass production techniques having cycle time constraints.